Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus and device manufacturing method makes use of a liquid confined in a reservoir between the projection system and the substrate. Bubbles forming in the liquid from dissolved atmospheric gases or from out-gassing from apparatus elements exposed to the liquid are detected and/or removed so that they do not interfere with exposure and lead to printing defects on the substrate. Detection may be carried out by measuring the frequency dependence of ultrasonic attenuation in the liquid and bubble removal may be implemented by degassing and pressurizing the liquid, isolating the liquid from the atmosphere, using liquids of low surface tension, providing a continuous flow of liquid through the imaging field, and/or phase shifting ultrasonic standing-wave node patterns.

This application claims priority from European patent application EP03253694.8, filed Jun. 11, 2003 and is a continuation-in-part of U.S.patent application Ser. No. 10/820,227, filed Apr. 8, 2004, each ofwhich is herein incorporated in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, such as a mask, may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g.comprising part of, one or several dies) on a substrate (e.g. a siliconwafer) that has a layer of radiation-sensitive material (resist). Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the projection beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in WO 99/49504, herebyincorporated in its entirety by reference. As illustrated in FIGS. 2 and3 a, liquid is supplied by at least one inlet IN onto the substrate,preferably along the direction of movement of the substrate relative tothe final element, and is removed by at least one outlet OUT afterhaving passed under the projection system. That is, as the substrate isscanned beneath the element in a −X direction, liquid is supplied at the+X side of the element and taken up at the −X side. FIG. 2 shows thearrangement schematically in which liquid is supplied via inlet IN andis taken up on the other side of the element by outlet OUT which isconnected to a low pressure source. In the illustration of FIG. 2 theliquid is supplied along the direction of movement of the substraterelative to the final element, though this does not need to be the case.Various orientations and numbers of in- and out-lets positioned aroundthe final element are possible, one example is illustrated in FIG. 3 ain which four sets of an inlet with an outlet on either side areprovided in a regular pattern around the final element.

SUMMARY

Unexpected disadvantages emerge from this new technology when comparedwith systems that do not have liquid in the exposure radiation path. Inparticular, despite the improved imaging resolution, the liquid tends todegrade the image quality in other respects. Accordingly, it would beadvantageous, for example, to improve the imaging performance of anapparatus having a liquid filling a space between the final element ofthe projection system and the substrate.

According to an aspect of the invention, there is provided alithographic projection apparatus comprising:

-   -   an illumination system arranged to condition a radiation beam;    -   a support structure configured to support a patterning device,        the patterning device being capable of imparting the radiation        beam with a pattern in its cross-section;    -   a substrate table configured to hold a substrate;    -   a projection system arranged to project the patterned radiation        beam onto a target portion of the substrate;    -   a liquid supply system configured to at least partly fill a        space between the projection system and the substrate with a        liquid; and    -   a bubble reduction device configured to reduce a size, a        concentration, or both of bubbles in the liquid, the bubble        reduction device comprising a bubble detector configured to        detect bubbles in the liquid.

An important source of image degradation in liquid immersion lithographymay be due to the scattering of imaging radiation from bubbles in theliquid. By reducing the size and concentration of these bubbles it maybe possible to reduce this scattering and the associated distortion ofthe image reaching the substrate, thereby reducing the frequency andmagnitude of defects in the printed pattern on the substrate. Bubblestypically form when dissolved gases from the atmosphere come out ofsolution due to a disturbance of some kind, or from out-gassing elementsof the lithographic apparatus, such as a photosensitive layer on thesubstrate. Bubbles thus formed may vary greatly in number density andsize distribution depending on the liquid, gases and disturbancesinvolved. Very fine bubbles tend to cause particular problems as theyare both difficult to detect and hard to remove using standard methodsand yet still influence the image formed on the substrate. For use inthe context of a typical lithographic apparatus, for example, bubblescontinue to degrade performance down to around 10 nm in diameter. Thebubble reduction device may comprise a bubble detector. The inclusion ofa bubble detector provides the possibility of feedback to the bubblereduction device, allowing adjustment and/or optimization of bubblereduction processes.

In an implementation, the bubble detector may comprise one or moreultrasonic transducers. These transducers may emit ultrasonic waves andreceive ultrasonic waves that are influenced by the presence of bubblesin the liquid within which they propagate. The information yielded bythe ultrasonic transducers may include information about thedistribution of bubble sizes as well as their number density.

The ultrasonic transducers may also measure the ultrasonic attenuationas a function of frequency. The advantage of this approach is that it ispossible to detect bubbles with dimensions very much smaller than thewavelength of the ultrasonic waves. Using only the amplitude of thesignal would restrict this measurement method to bubbles of the samesize or greater than the wavelength of the ultrasonic waves.

In an embodiment, the bubble reduction device may comprise a bubbleremoval device.

The bubble removal device may comprise a degassing device, the degassingdevice comprising an isolation chamber, wherein a space above liquid inthe isolation chamber is maintained at a pressure below atmosphericpressure encouraging previously dissolved gases to come out of solutionand be pumped away. A degassing process may dramatically reduce theoccurrence of bubbles due to dissolved atmospheric gases coming out ofsolution. Following the degassing process, the liquid is, in anembodiment, kept as isolated as possible from the normal atmosphere.

In an embodiment, the bubble removal device provides a continuous flowof liquid over the projection system and the substrate in order totransport bubbles out of the imaging field. This step is particularlyeffective for removing gases originating from out-gassing elements ofthe lithographic apparatus.

The bubble reduction device may pressurize the liquid above atmosphericpressure to minimize the size of bubbles and encourage bubble-forminggases to dissolve into the liquid.

The composition of the liquid may also be chosen to have a lower surfacetension than water. This may reduce the tendency of bubbles to stick tothe substrate where they may be particularly damaging to the image andwhere they tend to be resistant to removal measures. The tendency ofbubbles to stick to the substrate and other components may be reduced bycontrolling the surface finish in contact with the immersion liquid. Inparticular, the surface finish may be polished or arranged to have aminimal surface roughness, for example with a characteristic lengthscale of less than 0.5 μm.

The bubble reduction device may treat the liquid before it is introducedinto the space between the projection system and the substrate. Anadvantage of this approach is improved space considerations and libertyof design. These factors make it easier to treat liquid in bulk for usein a plurality of lithographic apparatuses or for use in a circulatorysystem or where the liquid is to be replaced on a frequent basis. Aftertreatment, the liquid may be protected from atmospheric gases by beingkept under vacuum or by being exposed only to a gas, such as nitrogen,argon or helium, which does not easily dissolve into the liquid.

The ultrasonic transducers of the bubble detector may be arranged in apulse-echo arrangement wherein the same transducer emits waves and,after reflection from a boundary, receives waves attenuated bypropagation through the liquid. An advantage of this arrangement is thatfewer transducers are used and it may be easier to arrange a relativelylong signal path through the liquid.

The bubble detector may comprise two spatially separated ultrasonictransducers, the first arranged to transmit, and the second to receivewaves. An advantage of this arrangement is that the signal received atthe receiving transducer may be easier to interpret and may suffer lessfrom anomalous signal loss caused, for example, by non-specularreflection from the boundary.

The bubble removal device may include two spatially separated ultrasonictransducers, arranged to produce ultrasonic standing-wave patternswithin the liquid that trap bubbles within nodal regions. The bubbleremoval device is arranged to displace the bubbles through the use of aphase adjusting device linked with the transducers, the phase adjustingdevice causing spatial shift of the nodal regions and of bubbles trappedwithin them. This process may be used to transport bubbles completely toone side of a liquid reservoir where they may be isolated and removedfrom the system.

In an embodiment, the ultrasonic transducers may operate at megasonicfrequencies (in the region of 1 MHz). Megasonic waves avoid some of thedisadvantages of conventional (lower frequency) ultrasonic waves such ascavitation and bubble collision with solid surfaces, which results insmall particles being dislodged and contaminating the liquid.

The bubble removal device may comprise an electric field generatorconfigured to apply an electric field to the liquid, the electric fieldbeing capable of dislodging bubbles attached to interfaces within theliquid. This feature may be particularly useful where an interface inquestion is the substrate, as bubbles attached here are at the focus ofthe lithographic projection apparatus and may therefore distort theimage more severely. The electric field lines are distorted in thevicinity of the bubble, which has a dielectric constant different fromthat of the surrounding liquid. This embodiment works on the basis thatwhen the bubble is close to, or in contact with, an interface, theelectric field distribution may be such as to force the bubble away fromthe surface and into the bulk of the liquid. Once in the bulk of theliquid, the bubble has a less detrimental effect on image quality, andmay also be more easily removed. This method is applicable even wherethe surface to which the bubble has attached is liquid-phobic andreduces the need to apply special liquid-philic coatings to thesubstrate.

The bubble removal device may comprise a heater configured toselectively control the temperature and therefore the size of bubblesaccording to their composition. By selecting to heat only the bubblesand not the surrounding liquid, it may be possible to minimizeunnecessary variations in the liquid temperature. Increasing thetemperature of the bubbles causes them to expand in size and thereforebecome easier to remove. The heater may comprise a microwave source,operating at frequencies that correspond to the resonant frequencies ofthe gas molecules forming the bubbles (commonly nitrogen and oxygen).Given the temperature sensitivity of the lithographic apparatus in theregion of the substrate, this method allows more extensive heating ofthe gas within the bubbles than would be the case if the liquid andbubbles had to be heated simultaneously. The result may be a more energyand time efficient method for removing bubbles from the liquid.

The bubble removal device may comprise a particle input deviceconfigured to introducing particles into the liquid, and a particleremoval device configured to remove the particles from the liquid. Thismechanism operates on the principle that, where the particles are chosenso that it is energetically or otherwise favorable, gas bubbles tend toattach to the surface of particles present in the liquid. Cumulatively,the particles present a large surface area to the liquid, whichincreases the chances of contact between particle and bubble. Thesurface in question may comprise the outer surface of the particles,and, where the particles are porous, the internal surface associatedwith the pores of the particles. Porous particles therefore provide alarger particle surface in contact with the liquid than non-porousparticles. This embodiment may be particularly effective when theparticles are arranged to have a surface that repels the liquid (i.e. asurface that has a high interface energy with the liquid). In the caseof a liquid comprising water, such a surface may be described ashydrophobic. This arrangement favors attachment of the bubbles as theyact to reduce the particle surface area in contact with the liquid, thusminimizing the surface energy. There may also be an electrostaticattraction between the bubble and particle, or other surfacecharacteristics of the particle that favor attachment of bubbles. Gasbubbles that become attached to the particles are removed from theliquid when the particles are removed from the liquid by the particleremoval device. The particle removal device may comprise a particlefilter. In general, the dimensions of the particle are chosen to makethem easy to remove, and this mechanism may provide efficient removal ofvery fine bubbles.

The bubble detector may comprise a light source, a light detector and alight comparator. The light source and the light detector may beconfigured so that light emitted by the source propagates between thesource and the detector through a portion of the liquid, the comparatorbeing arranged to detect changes in the proportion of the emitted lightthat arrives at the detector after propagation through a portion of theliquid. The presence of bubbles in the liquid causes the light to bescattered. Depending on the arrangement of the source and the detector,this scattering may cause an increase or a decrease in the signaldetected at the detector and may be analyzed to provide informationabout the population of bubbles. An advantage of this arrangement isthat it may be operated continuously, even when the projection apparatusis in normal operation. When bubbles occur, they may be detected at anearly stage and exposure may be suspended until the liquid is clearagain. This feature therefore may minimize lost time, and reduce thequantity of poorly exposed substrates that are produced.

According to a further aspect of the invention, there is provided alithographic projection apparatus comprising:

-   -   an illumination system arranged to condition a radiation beam;    -   a support structure configured to support a patterning device,        the patterning device being capable of imparting the radiation        beam with a pattern in its cross-section;    -   a substrate table configured to hold a substrate;    -   a projection system arranged to project the patterned radiation        beam onto a target portion of the substrate;    -   a liquid supply system configured to at least partly fill a        space between the projection system and the substrate with a        liquid; and    -   a detection system configured to detect impurities in the        liquid, including a light source, a light detector and a light        comparator, the light source and the light detector being        arranged so that light emitted by the source propagates between        the source and the detector through a portion of the liquid, the        comparator being arranged to detect changes in the proportion of        the emitted light that arrives at the detector after propagation        through a portion of the liquid.

A detection system may be arranged to detect particles in the liquidbetween the projection system and the substrate. Particles may beintroduced deliberately in order to control optical properties of theliquid and enhance the performance of the lithographic apparatus. Thismay be achieved, for example, by a fine suspension of quartz particles.In this case, the detection system may be used to verify that theparticles are present in the desired proportions. Alternatively oradditionally, damaging particles may enter the system by accident, suchas those that break away from surfaces in contact with the immersionliquid. In this case, the detection system may be used to detect theseparticles and initiate an alarm procedure when the particleconcentration and/or size distribution exceeds predetermined thresholds.Early detection of problems (whether a lack of desired particles or anexcess of undesirable particles) allows corrective action to be takenpromptly and may help to minimize loss of time and materials associatedwith substandard imaging.

According to a further aspect of the invention, there is provided adevice manufacturing method comprising:

-   -   providing a liquid to a space between a projection system of a        lithographic apparatus and a substrate;    -   projecting a patterned radiation beam using the projection        system, through the liquid, onto a target portion of a        substrate; and    -   detecting and reducing bubbles in the liquid.

According to a further aspect of the invention, there is provided alithographic projection apparatus comprising:

-   -   an illumination system arranged to condition a radiation beam;    -   a support structure configured to support a patterning device,        the patterning device being capable of imparting the radiation        beam with a pattern in its cross-section;    -   a substrate table configured to hold a substrate;    -   a projection system arranged to project the patterned radiation        beam onto a target portion of the substrate;    -   a liquid supply system configured to at least partly fill a        space between the projection system and the substrate with a        liquid; and    -   a liquid quality monitor capable of switching the operational        state of the projection apparatus between an active state and a        suspended state, the active state being selected when the liquid        quality is determined to be above a predefined threshold and the        suspended state being selected when the liquid quality is        determined to be below a predefined threshold state.

A liquid quality monitor may allow early detection of faults, and avoidunnecessary loss of time and material due to faulty exposure of thesubstrates. The predefined thresholds may be based on parameters such aslimits on the size and/or number distribution of bubbles as detected bya bubble detector. Alternatively, the predefined thresholds may relateto limits on the size and/or number distribution of other particles inthe liquid.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

Where reference is made to “ultrasonic” or “ultrasound”, unless statedotherwise, this is to be interpreted as relating to sound waves at anyfrequency greater than the upper limit of human perception, namely,greater than 20 kHz.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm).

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a projection beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

A patterning device may be transmissive or reflective. Examples of apatterning device include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned. In each example of a patterning device, thesupport structure may be a frame or table, for example, which may befixed or movable as required and which may ensure that the patterningdevice is at a desired position, for example with respect to theprojection system. Any use of the terms “reticle” or “mask” herein maybe considered synonymous with the more general term “patterning device”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion liquid or theuse of a vacuum. Any use of the term “projection lens” herein may beconsidered as synonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may be referred to below, collectively orsingularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts a liquid supply system for supplying liquid to the areaaround the final element of the projection system according to anembodiment of the invention;

FIG. 3 a depicts the arrangement of inlets and outlets of the liquidsupply system of FIG. 2 around the final element of the projectionsystem according to an embodiment of the invention;

FIG. 31 b depicts a liquid supply system according to an embodiment ofthe invention;

FIG. 4 depicts a liquid supply system with a bubble reduction deviceaccording to an embodiment of the invention;

FIG. 5 depicts two possible arrangements of ultrasonic transducers in abubble detection device according to two embodiments of the invention;

FIG. 6 depicts an arrangement of ultrasonic transducers and standingwaves in a bubble removal device according to an embodiment of theinvention;

FIG. 7 depicts a degassing device according to an embodiment of theinvention.

FIG. 8 depicts a liquid pressurization device according to an embodimentof the invention;

FIG. 9 depicts an embodiment of a bubble removal device showing a pairof protected electrodes and associated electric field generator;

FIG. 10 illustrates several different embodiments of the presentinvention with a different liquid supply system to that illustrated inFIGS. 2 and 3;

FIGS. 11 a and 11 b depict an embodiment of the bubble removal devicearranged to selectively heat bubbles via a microwave radiation source;

FIG. 12 depicts an embodiment of the bubble removal device comprising aparticle input device and a particle removal device;

FIG. 13 depicts an embodiment of the bubble detection device showing thelight source and light detector and an example trajectory for a beam oflight scattered from its path within the liquid through the projectionsystem to the light detector;

FIG. 14 depicts a larger scale view of the substrate region of thearrangement shown in FIG. 13, illustrating the introduction of lightfrom the light source into the region between the final element of theprojection system and the substrate, according to a first embodiment ofthe light source;

FIG. 15 depicts the same view as FIG. 14 but shows the introduction oflight from the light source into the region between the final element ofthe projection system and the substrate according to a second embodimentof the light source;

FIG. 16 depicts an embodiment of the bubble detection device comprisinga light source, detector, light comparator, liquid quality monitor andalarm; and

FIG. 17 depicts an arrangement of ultrasonic transducers in the regionof the final element of the projection system and the substrateaccording to an embodiment of the invention.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to aparticular embodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL for providing a        projection beam PB of radiation (e.g. UV radiation or DUV        radiation);    -   a first support structure (e.g. a mask table) MT for supporting        a patterning device (e.g. a mask) MA and connected to a first        positioning device PM for accurately positioning the patterning        device with respect to item PL;    -   a substrate table (e.g. a wafer table) WT for holding a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioning device PW for accurately positioning the        substrate with respect to item PL; and    -   a projection system (e.g. a refractive projection lens) PL for        imaging a pattern imparted to the projection beam PB by the        patterning device MA onto a target portion C (e.g. comprising        one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise an adjusting device AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

The projection beam PB is incident on the mask MA, which is held on themask table MT. Having traversed the mask MA, the projection beam PBpasses through the projection system PL, which focuses the beam onto atarget portion C of the substrate W. With the aid of the secondpositioning device PW and position sensor IF (e.g. an interferometricdevice), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the beam PB.Similarly, the first positioning device PM and another position sensor(which is not explicitly depicted in FIG. 1) can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval from a mask library, or during a scan. In general,movement of the object tables MT and WT will be realized with the aid ofa long-stroke module (coarse positioning) and a short-stroke module(fine positioning), which form part of the positioning device PM and PW.However, in the case of a stepper (as opposed to a scanner) the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizes aprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIGS. 2 and 3 a and FIG. 3 b depict liquid supply systems according toembodiments of the invention and have been described above and below,respectively. Other liquid supply systems may be employed according toembodiments of the invention including, without limitation, a bath ofliquid and a seal member as described below.

FIG. 4 shows a liquid supply system 1 and a bubble reduction device 3a/3 b according to an embodiment of the invention. The bubble reductiondevice 3 a/3 b may be located underneath the projection system 3a, orexterior to the imaging axis 3 b. The liquid supply system 1 suppliesliquid to a reservoir 13 between the projection system PL and thesubstrate W. In an embodiment, the liquid is chosen to have a refractiveindex substantially greater than 1 meaning that the wavelength of theprojection beam is shorter in the liquid than in gas (such as air) or avacuum, allowing smaller features to be resolved. It is well known thatthe resolution of a projection system is determined, inter alia, by thewavelength of the projection beam and the numerical aperture of thesystem. The presence of the liquid may also be regarded as increasingthe effective numerical aperture.

If the liquid has been exposed to the atmosphere, some atmospheric gasesmay be dissolved in the liquid. Disturbances of the fluid (in any way)may give rise to the formation of bubbles, which, depending on theliquid, gases and disturbances involved, may be very fine. Fine bubbles,down to around 10 nm in diameter, are very difficult to detect usingstandard methods but still interfere with the imaging performance of theexposure radiation, distorting the image and leading to printing defectson the substrate. Bubbles may also enter the reservoir 13 viaout-gassing from elements within the lithographic apparatus such as thephotosensitive layer on the substrate W when it is exposed.

The reservoir is bounded at least in part by a seal member 17 positionedbelow and surrounding the final element of the projection system PL. Theseal member 17 extends a little above the final element of theprojection system PL and the liquid level rises above the bottom end ofthe final element of the projection system PL. The seal member 17 has aninner periphery that at the upper end closely conforms to the step ofthe projection system or the final element thereof and may, e.g., beround. At the bottom, the inner periphery closely conforms to the shapeof the image field, e.g. rectangular but may be any shape.

Between the seal member 17 and the substrate W, the liquid may beconfined to the reservoir by a contact-less seal 16, such as a gas sealformed by gas, e.g. nitrogen, argon, helium or similar that do notreadily dissolve into the liquid, provided under pressure to the gapbetween the seal member 17 and the substrate W. Between the seal member17 and the projection system PL, the liquid is confined by sealingmembers 14, optionally to keep the liquid pressurized. Alternatively,the sealing members 14 may be omitted and the liquid confined bygravity.

The bubble reduction device 3 may comprise a bubble removal device. FIG.4 shows an aspect of the bubble removal device, wherein the liquid ismade to flow continuously past the projection system PL and substrate W.This action is particularly effective for transporting away bubbles fromgas originating within the reservoir 13, e.g. those arising due toout-gassing from the substrate W. Liquid is introduced to the reservoir13 through channels 23 formed at least partly in the seal member 17.These channels 23 may cooperate with channels for feeding thecontact-less seal 16, which may comprise one or more inlet and outletports for gas and/or liquid. For example, liquid may be sucked from theregion of the reservoir nearest the contact-less seal 16 by a gas outletport and arranged to feed the continuous flow.

The bubble reduction device 3 may comprise a bubble detection device 4.FIG. 5 shows two arrangements of ultrasonic transducers 5 a/5 b ofa-bubble detection device 4. The principle of detection used here isthat the ultrasonic wave amplitude will be attenuated due to Rayleighscattering from bubbles in the liquid. The ultrasonic attenuation is afunction of the size distribution and the number density of bubbles(i.e. the number per unit volume). In the left diagram, an ultrasonictransducer emits a pulse that, after passing through the immersionliquid and reflecting from a boundary within the reservoir (whetherreservoir 13 or some other reservoir, for example exterior to theimaging axis), is received by the same transducer 5 a. This arrangementof transducer 5 a is known as a “pulse-echo” arrangement. The pulse-echoarrangement is effective because it only uses a single transducer 5 aand it is relatively easy to have a large propagation path betweenemission and detection thus helping to maximize the sensitivity tobubbles. However, it is possible that anomalous reflections occurcausing loss of signal. The sampling rate may also be limited by thefact that it is necessary to wait for the return of a pulse beforeemitting a further pulse. Arranging the transducer 5 a so that it canemit and receive concurrently may obviate this problem. Anotherarrangement is shown on the right of FIG. 5, using two transducers 5 beach dedicated to either emitting or receiving ultrasonic waves. Here itis possible to emit rapid trains of pulses and the arrangement does notsuffer from anomalous reflection effects since the wave pulses traveldirectly between the transducers 5 b.

The attenuation is measured as a function of frequency in order todetect bubbles that are much smaller than the wavelength of theultrasonic signals. This may be done using broadband transducers andexcitations. Measuring attenuation at only a single frequency restrictsdetection to bubbles with diameters of the same order of size as orlarger than the wavelength of the ultrasonic signals.

FIG. 6 shows a further aspect of the bubble removal device according toan embodiment of the invention, wherein two ultrasonic transducers 5 cpowered by a signal generator 9 and phase shifted relative to each otherby a phase adjusting device 8 are arranged to produce a standing wavepattern 6 in the liquid between the faces of the transducers 5 c. FIG. 6shows a standing wave made up of interfering sine waves but the standingwaves may be of any periodic form (e.g square-wave or saw-tooth). Theupper diagram represents the arrangement at a first instant and thelower diagram the same arrangement at a later instant. Bubbles presentin the liquid (e.g. 2) tend to become localized near the nodal regions 7of the standing wave 6. The phase adjusting device 8 acts to shift thepositions of the nodes towards one or the other of the two ultrasonictransducers 5 c as shown by arrow 25. The trapped bubbles 2 move alongwith the moving nodes towards the transducer 5 c in question and aretherefore transported to an edge of a liquid reservoir. In FIG. 6, thismovement is to the left as indicated by the arrow 26 and thedisplacement of the sample trapped bubble 2 indicated by the displacedvertical broken lines that pass through the center of the trapped bubble2 at the two consecutive times. Once a certain concentration of bubbleshas accumulated near one transducer 5 c, the liquid in this region maybe isolated and removed from the reservoir, carrying the bubbles withit.

The bubble removal device may work using ultrasonic waves as describedin European patent application no. EP 03253694.8 hereby incorporated inits entirety by reference, or on similar principles using higherfrequency waves known as megasonic waves (about 1 MHz) which avoid someof the disadvantages of conventional ultrasonic waves (which may lead tocavitation and bubble collision with walls resulting in small particlesbreaking off the walls and contaminating the liquid). As an alternative,the ultrasonic energy may be controlled, even with lower frequencyultrasound, to reduce the likelihood or extent of bubble cavitation.Additionally, ultrasound may be used to cause coalescence of smallerbubbles into larger bubbles which rise more quickly and may be moreeasily removed.

Other bubble reduction devices are also possible, for example thosedescribed in the above mentioned European patent application as well asthe use of membranes perhaps in combination with a vacuum or by purgingthe liquid with a low solubility gas, such as helium. Membranes arealready used for removal of gases from liquids in fields such asmicroelectronics, pharmaceutical and power applications. The liquid ispumped through a bundle of semi-porous membrane tubing. The pores of themembrane are sized and the material chosen so that the liquid cannotpass through them but the gases to be removed can. Thus the liquid isdegassed. The process may be accelerated by applying to the outside ofthe tubing a low pressure. Liqui-Cel (™) membrane contactors availablefrom Membrana-Charlotte, a division of Celgard Inc. of Charlotte, N.C.,USA may be suitable for this purpose.

Purging with a low solubility gas is a known technique applied in highperformance chromatography to prevent gas bubble trapping in areciprocating pump head. When the low solubility gas is purged throughthe liquid, it drives out other gases, such as carbon dioxide andoxygen.

FIG. 7 shows the degassing device 10 of the bubble removal deviceaccording to an embodiment of the invention. The degassing device 10comprises an isolation chamber 11, which contains the liquid to bedegassed. The degassing device 10 may further comprise a pump 12arranged to extract gases from the isolation chamber 11 and, eventually,to achieve a low pressure state therein. In an implementation, theminimum pressure may be chosen to be greater than the saturated vaporpressure of the liquid being used so as to prevent boiling, e.g., around23 mbar for water at room temperature. Once under reduced pressure,gases dissolved in the liquid will leave solution and be pumped away bythe pump 12. Raising the temperature of the liquid may assist thisprocess. For example, working between 40 and 50° C. typically increasesthe degassing speed by about a factor of ten. When the degassing processis complete, i.e. when no further dissolved gas can be extracted fromthe liquid, the isolation chamber 11 may be isolated by closing doors 15located above the liquid. The liquid should remain isolated from theatmosphere until it is transferred into the reservoir 13 for use. Theliquid may be kept either under vacuum or under a gas that will noteasily dissolve into the liquid, such as nitrogen, argon or helium.

FIG. 8 shows a liquid pressurization device 22 that acts to pressurizethe reservoir liquid above atmospheric pressure according to anembodiment of the invention. High pressure has the effect of minimizingthe size of bubbles and encouraging bubbles to dissolve into the liquid.The apparatus shown in FIG. 8 comprises a piston 19 and a bore 21.Pushing the piston into the bore pressurizes the liquid. At its lowerend, a valve 18 is provided to allow transfer of the liquid, forexample, into the liquid supply system 1. For monitoring purposes, apressure gauge 20 is provided which may include a safety blow-off valve.

The bubble reduction device 3 may comprise elements both within thereservoir 13, as shown in FIG. 4, and outside the reservoir 13—see 3 aand 3 b respectively in FIG. 4. An advantage of having elements outsidethe exposure space 13 is that engineering considerations, such as theamount of space available or the allowable levels of vibrations and heatdissipation, are significantly relaxed. This fact not only makes itcheaper to design processing elements but also opens the possibility forbulk processing. Such bulk processing may allow a single station toprepare liquid for use in a number of lithographic apparatuses or toprovide a large quantity of conditioned liquid for use in a system wherethere is a continual throughput of liquid, or in a system where theliquid is changed on a frequent basis.

A bubble reduction device 3 located within the reservoir 13 isparticularly effective for dealing with bubbles that unavoidablyoriginate within the reservoir 13, such as from out-gassing.

The composition of the liquid may be chosen to have a lower surfacetension than water. This reduces the tendency of bubbles to stick to thesubstrate (particularly acute for small bubbles) where they may beparticularly damaging to the image and where they tend to be resistantto removal measures. This may be achieved by choosing a pure liquid witha lower surface tension or by adding a component to the liquid thatreduces its surface tension, such as a surfactant.

Bubbles attached to the surface of the substrate W are particularlydamaging because they are near the focus of the projection apparatus.The image is thus liable to be seriously distorted due to diffraction.An embodiment of the present invention provides a means for removingsuch bubbles and, more generally, bubbles attached to any interfaceswithin the immersion liquid. FIG. 9 illustrates one such embodiment, inthis case directed towards removing bubbles from the substrate W. Here,two electrodes 27 a and 27 b are arranged in the region between thefinal element of the projection system PL and the substrate W and areeach connected to terminals of an electrical power source 28.Alternatively, parts of the existing apparatus may be utilized aselectrodes. For example, the substrate W may form one electrode inpartnership with a second electrode such as 27 a. When energized, thisarrangement produces a uniform electric field substantially parallel tothe axis of the projection system PL which extends to the region ofliquid in close proximity to the target interface (e.g., thesubstrate/liquid interface). Bubbles have a dielectric constantdifferent from that of the surrounding liquid, which causes electricfield lines to be distorted in the region around the bubble. Whenbubbles are close to an interface such as the substrate (W), the fieldlines may be distorted in such a way that the bubble experiences aforce, which may be directed away from the surface in question and causethe bubble to deform and eventually break free from the surface andenter the bulk of the liquid. In the context of FIG. 9, the magnitude ofthe electric field may be arranged to overcome the pressure exerted onthe bubble due to the liquid located above it and other opposing forcesoriginating from factors such as surface tension. In an embodiment, thepotential difference between the electrodes 27 a and 27 b is 100 voltsDC. However, alternating voltage sources or a combination of alternatingand direct voltage sources may be used. An important parameter is theelectric field strength, which depends on the magnitude of the potentialdifference and the separation between the electrodes. Furthermore,non-uniform and differently oriented fields may also be effective. Thismechanism will be applicable even when the surface of the substrate W isliquid-phobic and there is a large energy barrier associated withdeforming the bubble and disconnecting it from the surface. This meansthat it is no longer necessary to specially treat the surface of thesubstrate W such as by coating it with a liquid-philic coating.

A number of design considerations should be taken into account. Theconductivity of the liquid should be carefully controlled. Inparticular, it should not be too high, because this will make itdifficult to create the electric field. Water with a resistivity ofroughly 0.8 to 18.2 MOhm*cm may be used for example. Also, theelectrodes 27 a and 27 b may be protected, in an implementation, frombreakdown by isolating material 29 to prevent electrolysis andsubsequent material breakdown. The conductivity and/or dielectricpermittivity of the electrodes themselves should be high in comparisonto the immersion liquid. One consequence of this will be to ensure thatthere is no appreciable fall in potential within the conductor material,which may help produce a uniform field between the electrodes.

Electrical forces may also cause adhesion between bubbles and solidparticles dispersed in liquid. Bubbles in a liquid have, on theirsurface, an electrokinetic (or zeta) potential which results in apotential difference between the surface of the bubble and the fullydisassociated ionic concentration in the body of the liquid. This alsoapplies to small particles.

According to an embodiment of the present invention, a power source orvoltage supply V (or charge, voltage, electrical field or potentialdifference generator or supply) may be used to apply an electricalpotential to one or more objects of the immersion apparatus. Theprinciple of operation is that if repulsion is needed a potentialdifference between the fully disassociated ionic concentration of theliquid and the object is generated, which is of the same polarity as thepotential difference between the fully disassociated ionic concentrationin the body of the liquid and the surface of the bubble. If attractionbetween the object and the bubble is needed, the potential differencesshould have the same polarity. In this way forces may be generated onthe bubbles towards or away from the objects (electrodes) which are incontact with the immersion liquid.

In FIG. 10, several different objects have a potential or charge appliedto them. This embodiment will work with only one such object and alsowith any combination of objects and indeed other objects to those notillustrated could be also or alternatively be used.

In pure water, which is the most promising candidate for use as animmersion liquid at 193 nm projection beam wavelength, it has been foundthat the surface potential of μm bubbles is about −50 mV. This potentialwill vary with bubble size and also with type of immersion liquid.However, the same principles as described here may be used for otherimmersion liquids and bubble sizes and the embodiments of the inventionare fully applicable to those. Additives may be added to the immersionliquid to change the effect of the potential. CaCl₂ or NaCl are suitablecandidate additions for this purpose.

In FIG. 10 six different objects are illustrated to which a potential orvoltage or charge could be applied. In an embodiment, the objects are incontact with the immersion liquid, though in principle this is notnecessary. One of these is the substrate W which is, in an embodiment,charged to the same polarity of electrical potential as the electricalpotential of the surface of the bubbles. In this way the bubbles have aforce on them directly away from the substrate W so that their effect onthe projected image is minimized. In combination with a negativepotential on the substrate W, or by itself, the final element of theprojection system or an object 50 close to the final element of theprojection system PL may be charged to a potential opposite in polarityto the potential of the surface of the bubbles. This will have theeffect of attracting the bubbles towards the final element of theprojection system and thereby away from the substrate. The shape of theobject 50 (electrode) close to the final element of a projection systemPL could be any shape. It could be plate like or could be annular sothat the projection beam PB passes through the centre of electrode 50.

Alternatively or additionally, an object to be charged or have a voltageapplied to it could be attached to a surface of the seal member 17. InFIG. 10, this object is attached to the inner surface of the seal member17. As illustrated, two electrodes 52, 54 are present each on oppositesides of the seal member 17 and charged to opposite potentials. In thisway the bubbles could be drawn to one or other of the objects, perhapsin the direction of an immersion liquid outlet. Alternatively oradditionally, one or more objects may be provided around the inner sideof the seal member 17 (in contact with the immersion liquid) whichis/are charged to a potential with a polarity different to the polarityof the potential of the surface of the bubbles. In this way bubbles inthe immersion liquid in the space 36 between the final element of theprojection system PL and the substrate W will be drawn away from theoptical axis of the apparatus thereby leaving the path of the projectionbeam PB to the substrate W substantially unhindered by bubbles.

Another place to use this embodiment is upstream of the space 36 betweenthe final element of the projection system PL and the substrate W in theliquid supply system. In this case, as the immersion liquid passes alongconduits 56 and through a housing 58, oppositely charged and opposingplates 62, 64 produce a force on the bubbles which is effective to movethe bubbles, when the immersion liquid is in the space 36, further awayfrom the substrate W than they would be without the application of theelectrical field upstream of the space 36. The immersion liquid with ahigh concentration of bubbles i.e. near to the electrode 64, could evenbe removed and not supplied to the space 36. The removed liquid could besubjected to a bubble removal process before being recycled in theliquid supply system.

In all of the above examples, the higher the voltage applied by thevoltage generator V the greater the force on the bubbles. The potentialon the object(s) should not be so high as to cause disassociation of theimmersion liquid but should be high enough to provide an effective forceon the bubbles. For an immersion liquid comprised mainly of water,typical potential differences applied to the object(s) according to thisembodiment are 5 mV to 5 V, optionally 10 mV to 500 mV. In anembodiment, an electrical field of 5 mV/mm to 500 mV/mm due to theapplication of the potential could be provided.

FIG. 11 illustrates an embodiment of the bubble removal device thatbenefits from a significantly enhanced bubble removal rate without undueinfluence on the immersion liquid. The improved removal rate is achievedby increasing the size of the bubbles in the immersion liquid byheating. The increased bubble size renders them more responsive to mostmethods of bubble removal. This is achieved without adverse heating.effects in the immersion liquid, or surrounding temperature sensitivecomponents, through the use of a microwave radiation source 30,producing radiation that couples only to the gas within the bubblesthemselves and not to the immersion liquid itself. FIG. 11 a, whichshows a schematic magnified view of the immersion liquid, illustrateshow the process operates. Microwave photons 32 are absorbed by anexample bubble 31 a at temperature T1, which is then heated to become alarger bubble 31 b at temperature T2. Once the temperature of the bubblehas been elevated above that of the surrounding immersion liquid, somerise in the temperature of the immersion liquid will inevitably occur inthe immediate vicinity of each bubble. However, the combined heatcapacity of the bubbles and thermal conductivity of the immersion liquidare likely to be small enough that heating of the immersion liquid maybe kept within acceptable limits. In general, the frequency componentsof the microwave radiation are chosen to correspond with resonantfrequencies or excitation modes of species present in the bubbles. Formany cases of interest, a large fraction of the gas forming the bubbleswill be nitrogen and oxygen, in which case the resonant modes of thesemolecules will dictate the microwave frequencies to use.

FIG. 12 illustrates another embodiment of the bubble removal device.Here, a particle input device 33 introduces into the immersion liquidparticles that act to attract bubbles to their surface. The particlesmay be mixed with the immersion liquid either by natural dispersion ordeliberate agitation. The particles may be left in the immersion liquidfor a period determined according to the concentration of bubbles. Forexample, if the bubble concentration is very high, the particles willbecome saturated quickly and will need to be refreshed after arelatively short time. If, on the other hand, the bubble concentrationis low, the particles may remain active for a much longer time. Once theactivity of the particles, or alternatively the bubble concentration,has fallen below a certain threshold level, the particles may be removedfrom the liquid by a particle removal device 34, which may comprise forexample a particle filter. According to the embodiment of FIG. 11, theparticle input device 33 and particle removal device 34 are coupled tothe channels 23 for circulating the immersion liquid through the region36 through a circuit indicated by the arrows 37 and 38. The circuit inquestion may be closed, as indicated by arrows 38, or involve input andoutput to a main, or other, liquid supply as indicated by arrows 37. Theused particles may be treated in a particle recycling device 35 toremove the gas bubbles from the particles. This de-gassing process maybe achieved, for example, by pumping on a solution containing theparticles or by pumping directly on the particles themselves. The cleanparticles may then be reintroduced to the immersion liquid via theparticle input device 33 where they will again act effectively to trapbubbles.

In an embodiment, the particles are arranged to have surfacecharacteristics that encourage bubbles to attach to the surface, forexample in order to lower their surface energy. In addition, theparticles may be arranged to have as large a surface area as possible.This may be achieved by using porous particles such that bubbles mayattach on surfaces within the interior of the particles. Generally thisparameter may be varied by controlling the size and number distribution,and porosity of the particles. A balance may need to be achieved in poresize because although finer pores may provide the greatest additionalsurface area, they will exclude bubbles that are of a similar order ofsize or that are large in comparison with the pores (the pores may alsobe blocked by such bubbles). Many different particle compositions may beused, for example silica, zeolites, alumina, activated carbon, or acarbon molecular sieve. Certain polymer compositions may also be used.The particle size is a less critical factor (compared with the surfacearea) and a typical size range may be 5 to 1000 μm diameter.

In FIG. 12, the particle input device 33 and the particle removal device34 are both located outside of the region 36. However, these componentsmay also be arranged to add and remove particles directly within thisregion.

Another method for bringing the particles into the liquid is theoccasional use of ultrasonic agitation in combination with non-degassedliquid. Due to cavitation of the bubbles, particles will be releasedfrom solid surfaces exposed to the liquid.

FIG. 13 shows a schematic representation of a section of thelithographic projection apparatus between the mask MA and the substrateW. This diagram shows several possible embodiments of the inventionwherein either the bubble detection device or a detection system isarranged to propagate light between a light source 39 and a detector 40.The presence of bubbles (in the case of the bubble detection device)and/or particles (in the case of the detection system) is establishedvia an increase or decrease in the intensity of light reaching thedetector 40, caused by light scattering from bubbles and/or particleswithin the liquid. FIG. 13 shows one possible arrangement, with thelight source 39 arranged via an optical fiber 41 to direct light raysinto the immersion liquid. Light propagates through the liquid and, ifbubbles and/or particles are present, may scatter from them. An examplepath for a scattered ray is shown by the arrows 42, showing propagationthrough the projection system to the detector 40. In an embodiment, awavelength is chosen such that the photoresist is insensitive to thelight. FIGS. 14 and 15 show magnified views of the substrate regionshowing how the light is fed into the immersion liquid. In FIG. 14, theoptical fiber 41 is fed through the seal member 17 and light makes itsway into the region 36 either directly or after a number of reflections.FIG. 15 shows an alternative arrangement whereby light is introducedbetween the substrate W and the seal member 17. In FIGS. 14 and 15,light is shown (by arrows 43 a and 43 b) entering from a singledirection and traversing the region 36 horizontally. However, light maybe fed into the liquid from any direction and take various pathsincluding paths comprising one or more reflections off the final elementof the projection system PL and/or the substrate W. According to theembodiment illustrated in FIGS. 13 to 15, the signal strength detectedat the light detector will increase as the concentration of bubblesand/or particles in the liquid increases due to the overall increase inscattering. However, the light source 39 and detector 40 may be arrangedso that increased scattering leads to a decrease in the signal strengtharriving at the detector 40. As a further variation, the optical fiber41 may connect to both an illumination source and a detector, thepresence of bubbles and/or particles being detected by a change in theamount of light being reflected back into the optical fiber 41.

The arrangement illustrated in FIGS. 13 to 15, which in general may bedescribed as a light scatterometer, has the advantage of allowingcontinuous and non-disruptive monitoring of the concentration of bubblesand/or particles in the immersion liquid. FIG. 16 illustratesschematically how the arrangement may be realized, with the light source39 and detector 40 interacting with a light comparator 44. The lightcomparator 44 compares the light emitted by the light source 39 and thesignal level arriving at the detector 40, and, depending on thearrangement of source and detector, determines information about thepopulation of bubbles and/or particles present in the immersion liquid.

The light comparator 44 may interact with a liquid quality monitor 45,which may be realized by a suitably programmed computer. The liquidquality monitor 45 may be arranged to ensure that the liquid is alwaysat a suitable level of cleanliness to ensure that the quality of theimage being written to the substrate W does not fall below a minimumthreshold level. The liquid quality monitor 45 may take into account, inaddition to the concentration of bubbles and/or particles, other factorssuch as the chemical composition of the liquid. The liquid qualitymonitor 45 may in turn be coupled to an alarm system 46 that causes thesystem to be shut down from an active state to a suspended state, orother appropriate action to be taken, when the state of the immersionliquid falls outside predefined parameters. This early reaction toproblems in the liquid allows corrective action to be taken promptly,and may also minimize the loss of materials and time associated withsubstandard exposures caused by low quality immersion liquid.

The imaging performance of the lithography system may also be affectednegatively (causing stray light, for example) by contamination on thebottom part of the projection system PL. Such contamination may include,for example, the formation of salts arising primarily from the resistchemicals or oxides such as SiO₂. The contamination may be reduced bymechanical or chemical cleaning, but such procedures involve expensivestoppages and service man hours, are not always completely effective andrisk damage to the projection system. According to an embodiment of thepresent invention described above, one or more ultrasonic transducersare provided to detect or remove bubbles from the immersion liquid.These devices may also be oriented and configured to removecontamination from the final element of the projection system PL and thesubstrate W or substrate table WT.

FIG. 17 shows one possible arrangement, wherein ultrasonic transducers47 are located on the seal member 17 and may couple directly to theliquid between the final element of the projection system PL and thesubstrate W. To minimize the risk of altering the position of theprojection system itself during cleaning, the transducers 47 may bemechanically isolated from, or at least in damped connection with, theseal member 17. For example, the transducers 47 may be located nearby,rather than on, the seal member 17. Alternatively, the device connectionto the projection system PL may be mechanically released when the highfrequency is generated. In the context of projection system or substratetable cleaning, a wide variety of high frequency generators may be usedthat produce ultrasonic waves resonant with the immersion liquid. Inpractice, the ultrasonic projection system and substrate table cleaningaction may be implemented automatically and be arranged to cycle on andoff according to the rate of contamination.

Another immersion lithography solution which has been proposed is toprovide the liquid supply system with a seal member which extends alongat least a part of a boundary of the space between the final element ofthe projection system and the substrate table. The seal member issubstantially stationary relative to the projection system in the XYplane though there may be some relative movement in the Z direction (inthe direction of the optical axis). A seal is formed between the sealmember and the surface of the substrate. In an embodiment, the seal is acontactless seal such as a gas seal. Such a system is disclosed in, forexample, U.S. patent application Ser. No. US 10/705,783, herebyincorporated in its entirety by reference.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

In European patent application no. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two substrate tables for supporting thesubstrate. Leveling measurements are carried out with a substrate tableat a first position, without immersion liquid, and exposure is carriedout with a substrate table at a second position, where immersion liquidis present. Alternatively, the apparatus can have only one substratetable moving between the first and second positions.

Embodiments of the present invention may be applied to any immersionlithography apparatus and any liquid supply system (including relevantparts thereof), in particular, but not exclusively, to any of thoseliquid supply systems mentioned above and the bath of liquid asdescribed above.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic projection apparatus comprising: an illuminationsystem arranged to condition a radiation beam; a support structureconfigured to support a patterning device, the patterning device beingcapable of imparting the radiation beam with a pattern in itscross-section; a substrate table configured to hold a substrate; aprojection system arranged to project the patterned radiation beam ontoa target portion of the substrate; a liquid supply system configured toat least partly fill a space between the projection system and thesubstrate with a liquid; and a bubble reduction device configured toreduce a size, a concentration, or both of bubbles in the liquid, thebubble reduction device comprising a bubble detector configured todetect bubbles in the liquid.
 2. An apparatus according to claim 1,wherein the bubble detector comprises an ultrasonic transducerconfigured to measure attenuation of ultrasonic waves in the liquid soas to obtain information about bubbles present in the liquid.
 3. Anapparatus according to claim 2, wherein the ultrasonic transducer isconfigured to measure ultrasonic attenuation as a function of frequency.4. An apparatus according to claim 2, wherein an ultrasonic transduceris arranged in a pulse-echo configuration, the transducer acting both totransmit ultrasonic waves and, after reflection, to receive ultrasonicwaves that have been attenuated during propagation along a path throughthe liquid.
 5. An apparatus according to claim 2, wherein the bubbledetector comprises two spatially separated ultrasonic transducers, thefirst arranged to transmit ultrasonic waves, and the second to receiveultrasonic waves that have been attenuated during propagation along apath through the liquid between the two transducers.
 6. An apparatusaccording to claim 1, wherein the bubble reduction device comprises abubble removal device.
 7. An apparatus according to claim 6, wherein thebubble removal device comprises a degassing device, the degassing devicecomprising an isolation chamber, wherein a space above liquid in theisolation chamber is maintained at a pressure below atmospheric pressureto encourage previously dissolved gases in the liquid to come out ofsolution and be pumped away.
 8. An apparatus according to claim 6,wherein the bubble removal device is configured to provide a continuousflow of liquid over the projection system and the substrate to transportbubbles in the liquid out of the space between the projection system andthe substrate.
 9. An apparatus according to claim 6, wherein the bubbleremoval device includes two spatially separated ultrasonic transducers,arranged to produce ultrasonic standing-wave patterns within the liquidwhich trap bubbles within nodal regions, the bubble removal device beingarranged to displace the bubbles through the use of a phase-adjustingdevice linked with the transducers, the phase-adjusting device causingspatial shift of nodal regions and bubbles trapped therein.
 10. Anapparatus according to claim 6, wherein the bubble removal devicecomprises an electric field generator configured to apply an electricfield to the liquid, the electric field being capable of dislodgingbubbles attached to the substrate.
 11. An apparatus according to claim6, wherein the bubble removal device comprises a heater configured toselectively control the temperature and therefore size of bubbles of aparticular composition.
 12. An apparatus according to claim 11, whereinthe heater comprises a microwave source.
 13. An apparatus according toclaim 6, wherein the bubble removal device comprises a particle inputdevice configured to introduce particles into the liquid, and a particleremoval device configured to remove the particles from the liquid. 14.An apparatus according to claim 13, wherein the particles comprise asurface with characteristics that encourage bubbles to attach thereto.15. An apparatus according to claim 1, wherein the bubble reductiondevice comprises a liquid pressurization device configured to pressurizethe liquid above atmospheric pressure to minimize the size of bubblesand encourage bubble-forming gases to dissolve into the liquid.
 16. Anapparatus according to claim 1, wherein the composition of the liquid ischosen to have a lower surface tension than water.
 17. An apparatusaccording to claim 1, wherein the bubble reduction device is configuredto treat the liquid before it is supplied to the space between theprojection system and the substrate.
 18. An apparatus according to claim17, wherein the treated liquid is kept in a sealed container, excessspace in the sealed container comprising one or more of the following:nitrogen gas, argon gas, helium gas or a vacuum.
 19. An apparatusaccording to claim 1, wherein the bubble detector comprises a lightsource, a light detector and a light comparator, the light source andthe light detector being arranged so that light emitted by the sourcepropagates between the source and the detector through a portion of theliquid, the comparator being arranged to detect changes in theproportion of the emitted light that arrives at the detector afterpropagation through a portion of the liquid.
 20. A lithographicprojection apparatus comprising: an illumination system arranged tocondition a radiation beam; a support structure configured to support apatterning device, the patterning device being capable of imparting theradiation beam with a pattern in its cross-section; a substrate tableconfigured to hold a substrate; a projection system arranged to projectthe patterned radiation beam onto a target portion of the substrate; aliquid supply system configured to at least partly fill a space betweenthe projection system and the substrate with a liquid; and a detectionsystem configured to detect impurities in the liquid, including a lightsource, a light detector and a light comparator, the light source andthe light detector being arranged so that light emitted by the sourcepropagates between the source and the detector through a portion of theliquid, the comparator being arranged to detect changes in theproportion of the emitted light that arrives at the detector afterpropagation through a portion of the liquid.
 21. An apparatus accordingto claim 20, wherein the detection system is configured to detectparticles in the liquid between the projection system and the substrate.22. A device manufacturing method comprising: providing a liquid to aspace between a projection system of a lithographic apparatus and asubstrate; projecting a patterned radiation beam using the projectionsystem, through the liquid, onto a target portion of a substrate; anddetecting and reducing bubbles in the liquid.
 23. A method according toclaim 22, comprising measuring attenuation of ultrasonic waves in theliquid so as to obtain information about bubbles present in the liquid.24. A method according to claim 23, wherein ultrasonic attenuation ismeasured as a function of frequency.
 25. A method according to claim 22,comprising removing bubbles from the liquid.
 26. A method according toclaim 25, wherein removing bubbles from the liquid comprises degassingthe liquid by maintaining a space of the liquid at a pressure belowatmospheric pressure to encourage previously dissolved gases in theliquid to come out of solution and be pumped away.
 27. A methodaccording to claim 25, wherein removing bubbles comprises providing acontinuous flow of liquid over the projection system and the substrateto transport bubbles in the liquid out of the space between theprojection system and the substrate.
 28. A method according to claim 25,wherein removing bubbles comprises producing ultrasonic standing-wavepatterns within the liquid which trap bubbles within nodal regions anddisplacing the bubbles through a spatial shift of the nodal regions andbubbles trapped therein.
 29. A method according to claim 25, whereinremoving bubbles comprises applying an electric field to the liquid, theelectric field being capable of dislodging bubbles attached to thesubstrate.
 30. A method according to claim 25, wherein removing bubblescomprises selectively controlling the temperature and therefore size ofbubbles of a particular composition.
 31. A method according to claim 30,wherein the temperature is controlled using microwave radiation.
 32. Amethod according to claim 25, wherein removing bubbles comprisesintroducing particles into the liquid and removing the particles fromthe liquid.
 33. A method according to claim 32, wherein the particlescomprise a surface with characteristics that encourage bubbles to attachthereto.
 34. A method according to claim 22, wherein reducing bubblescomprises pressurizing the liquid above atmospheric pressure to minimizethe size of bubbles and encourage bubble-forming gases to dissolve intothe liquid.
 35. A method according to claim 22, wherein the compositionof the liquid is chosen to have a lower surface tension than water. 36.A method according to claim 22, wherein reducing bubbles, detectingbubbles or both occurs prior to the liquid is provided to the spacebetween the projection system and the substrate.
 37. A method accordingto claim 36, wherein the treated liquid is kept in a sealed container,excess space in the sealed container comprising one or more of thefollowing: nitrogen gas, argon gas, helium gas or a vacuum.
 38. A methodaccording to claim 22, wherein reducing bubbles, detecting bubbles orboth occurs as the liquid is provided to or while the liquid is in thespace between the projection system and the substrate.
 39. A methodaccording to claim 22, wherein detecting bubbles comprises propagatinglight through a portion of the liquid and detecting changes in theproportion of emitted light that arrives at a detector after propagationthrough a portion of the liquid.
 40. A lithographic projection apparatuscomprising: an illumination system arranged to condition a radiationbeam; a support structure configured to support a patterning device, thepatterning device being capable of imparting the radiation beam with apattern in its cross-section; a substrate table configured to hold asubstrate; a projection system arranged to project the patternedradiation beam onto a target portion of the substrate; a liquid supplysystem configured to at least partly fill a space between the projectionsystem and the substrate with a liquid; and a liquid quality monitorcapable of switching the operational state of the projection apparatusbetween an active state and a suspended state, the active state beingselected when the liquid quality is determined to be above a predefinedthreshold and the suspended state being selected when the liquid qualityis determined to be below a predefined threshold state.